The present invention relates to a diesel fuel stabiliser. In particular the present invention relates to a stabiliser for a diesel fuel containing a cetane improver.
As discussed in U.S. Pat. No. 5,482,518 fuel ignition in diesel engines is achieved through the heat generated by air compression, as a piston in the cylinder moves to reduce the cylinder volume during the compression stroke. In the engine, the air is first compressed, then the fuel is injected into the cylinder; as the fuel contacts the heated air, it vaporises and finally begins to burn as the self-ignition temperature is reached. Additional fuel is injected during the compression stroke and the fuel burns almost instantaneously, once the initial flame has been established. Thus, a period of time elapses between the beginning of fuel injection and the appearance of a flame in the cylinder. This period is commonly called xe2x80x9cignition delayxe2x80x9d and must be relatively short in order to avoid xe2x80x9cdiesel knockxe2x80x9d.
A major contributing factor to diesel fuel performance and the avoidance of xe2x80x9cdiesel knockxe2x80x9d is the cetane number of the diesel fuel. Diesel fuels of higher cetane number exhibit a shorter ignition delay than do diesel fuels of a lower cetane number. Therefore, higher cetane number diesel fuels are desirable to avoid diesel knock. Most diesel fuels possess cetane numbers in the range of about 40 to 55 and a sulfur content of about 500 ppm and less. A correlation between ignition delay and cetane number has been reported in xe2x80x9cHow Do Diesel Fuel Ignition Improvers Workxe2x80x9d Clothier, et al., Chem. Soc. Rev, 1993, pg. 101-108.
Cetane improvers have been used for many years to improve the ignition quality of diesel fuels. The use of cetane improvers is increasing due to the increased demand for diesel fuel which has resulted in a widening of the fraction recovered, the so called middle distillate fraction, and the lower natural cetane number of diesel base stocks caused by more severe refining of crude oil and the effort made to produce low emission diesel.
Many types of additives have been prepared and evaluated to raise the cetane number of diesel fuel. Such additives include peroxides, nitrates, nitrites, azo compounds and the like.
Alkyl nitrates such as amyl nitrate, hexyl nitrate and mixed octyl nitrates have been used commercially with good results. Other nitrates such as 2-methyl-2-nitropropyl nitrate (U.S. Pat. No. 4,536,190) have been suggested as cetane improvers but found shock sensitive. However, it is generally accepted that organic nitrates, more specifically the commercial 2-ethylhexyl nitrate, are the most cost-effective additives to improve cetane number of diesels. 2-ethylhexyl nitrate is available from the Associated Octel Company Limited under the brand name Cl-0801.
It is known that 2-ethylhexyl nitrate (2-EHN) has a de-stabilising effect on many diesel fuels when subjected to high temperature test conditions, such as ASTM D6468-99, Standard Test Method for High Temperature Stability of Distillate fuels. D6468 is a test that involves aging fuel for 90 or 180 minutes at a temperature of 150xc2x0 C. This is a temperature above the auto-decomposition temperature of cetane improvers such as 2-EHN, and the decomposition products promote instability of diesel fuels.
Recent data by Bacha, et al. have demonstrated that high temperature stability as measured in D6468 is related to on-road performance of diesel trucks. Because of these findings, the National Council of Weights and Measures has chosen to specify test results developed from ASTM D6468 as a partial basis for defining xe2x80x9cpremiumxe2x80x9d diesel fuel in the United States. There is therefore a desire to overcome the problems of diesel destabilisation by cetane improvers, in particular by 2-EHN.
The prior art has provided some additives which are normally used to improve storage stability and will give some improvement in ASTM D6468. EP-A-0947577 teaches tertiary alkyl primary amines may stabilise diesel fuel containing cetane number improver.
The present invention alleviates the problems of the prior art
In a one aspect the present invention provides use of a thermal stabiliser for increasing the thermal stability of a cetane improver in a fuel composition comprising (i) a fuel, and (ii) the cetane improver, wherein the thermal stabiliser is a compound of the formula Hxe2x80x94R1 wherein H is a group comprising a heterocyclic group and R1 is a hydrocarbyl group having from 10 to 200 carbons.
In a further aspect the present invention provides a fuel composition comprising (i) a fuel (ii) a cetane improver (iii) a thermal stabiliser, wherein the thermal stabiliser is a compound of the formula Hxe2x80x94R1 wherein H is a group comprising a heterocyclic group and R1 is a hydrocarbyl group having from 10 to 200 carbon.
The present inventors have found that compounds such as those defined in the present claims, for example polyisobutenyl succinimide (PIBSI), are surprisingly effective in stabilising cetane improvers in diesel fuel. In particular the present compounds are effective in reducing the thermal degradation of cetane improvers such as 2-EHN.
The present inventors have found and have demonstrated in the attached examples that the present thermal stabilisers such as PIBSI effectively stabilise cetane improvers in diesel and in some aspects are provide improved stabilisation when compared to the stabilisers of the prior art, for example the stabilisers of EP-A-0947577.
Furthermore, in aspects of the invention, for example when PIBSI is used as a stabiliser, the thermal stabiliser has a further functionalityxe2x80x94it is an effective detergent to improve injector cleanliness, for example in an L-10 Cummins diesel engine (a test is commonly used in the USA) and other diesel engine detergency tests.
In the present specification by the term xe2x80x9chydrocarbyl groupxe2x80x9d it is meant a group comprising at least C and H and may optionally comprise one or more other suitable substituents. Examples of such substituents may include halo-, alkoxy-, nitro-, a hydrocarbon group, an N-acyl group, a cyclic group etc. In addition to the possibility of the substituents being a cyclic group, a combination of substituents may form a cyclic group. If the hydrocarbyl group comprises more than one C then those carbons need not necessarily be linked to each other. For example, at least two of the carbons may be linked via a suitable element or group. Thus, the hydrocarbyl group may contain hetero atoms. Suitable hetero atoms will be apparent to those skilled in the art and include, for instance, sulphur, nitrogen and oxygen.
Thermal Stabiliser
R1 Group
Preferably R1 is a branched or straight chain alkyl group. Preferably R1 is a branched alkyl group.
R1 may be a C10-C200 hydrocarbon group. R1 is preferably a C30-C80 group and more preferably a polyisobutene (PIB).
Conventional PIBs and so-called xe2x80x9chigh-reactivityxe2x80x9d PIBs (see for example EP-A-0 565 285) are suitable for use in the invention. High reactivity in this context is defined as a PIB wherein at least 50%, preferably 70% or more, of the terminal olefinic double bonds are of the vinylidene type, for example the GLISSOPAL compounds available from BASF.
Preferably R1 has a molecular weight of from 200 to 2000, more preferably 260 to 1000, for example about 260, 320, 350, 550, 750, 780 or 1000, more preferably from 750 to 780.
Preferably R1 is a PIB having a molecular weight of from 200 to 2000, more preferably 260 to 1000, for example about 260, 320, 350, 550, 750, 780 or 1000, more preferably from 750 to 780.
Hxe2x80x94Heterocyclic Group
Preferably the heterocyclic group is a hydrocarbyl ring at least one member of which is selected from N, S and O. The ring portion of such a ring is referred to as a heterocyclic ring. Preferably at least one member of the hydrocarbyl ring is N.
Preferably the heterocyclic ring has from 4 to 10 members, more preferably from 4 to 6 members, yet more preferably 5 members.
Preferably the heterocyclic ring is substituted. Preferably the heterocyclic ring is substituted with one or more=O groups.
Preferably the heterocyclic ring is of the formula 
R2 may be selected from hydrogen, C1-20 straight, branched or substituted alkyl, or polyamine. The alkyl may be methyl, ethyl, butyl. The polyamine may be ethylenediamine, diethylenetriamine, triethylenetetraamine, tetraethylenepentaamine, pentaethylenehexamine, dimethylaminopropylamine, aminoethylethanolamine, and other commercially available materials which comprise complex mixtures of polyamines.
Preferably the heterocyclic group comprises linker via which the heterocyclic ring is attached to R1. Preferably the linker group is a C1-5 alkyl group, more preferably a C1-3 alkyl group, yet more preferably a xe2x80x94CH2xe2x80x94 group.
Preferably Nxe2x80x94R2 is a residue of a long chain polyalkylenepolyamine.
Stabiliser Hxe2x80x94R1 
The compound Hxe2x80x94R1 may in the form of a xe2x80x9cdimerxe2x80x9d or may capped with a further H group. Thus in one aspect the compound Hxe2x80x94R1 may be present in the form of a compound of the formula Hxe2x80x94R1xe2x80x94R1xe2x80x94H or Hxe2x80x94R1xe2x80x94H. In further aspects, the thermal stabilser
Preferably the thermal stabiliser is polyisobutene succinimide monotetraethylene pentaamine. More preferably the thermal stabiliser is polyisobutene succinimide monotetraethylene pentaamine having a PIB molecular weight of from 200 to 2000, more preferably 260 to 1000, for example about 260, 320, 350, 550, 750, 780 or 1000, more preferably from 750 to 780. In a highly preferred embodiment the thermal stabiliser is 780-polyisobutene succinimide monotetraethylene pentaamine.
Cetane Improver
Preferably the cetane improver is 2-ethylhexyl nitrate.
Fuel
Preferably the fuel is a diesel fuel
Composition
The thermal stabiliser compound of the present invention can be combined with other additives to give other improvements, in an additive package/thermal stabiliser composition. The thermal stabiliser composition may comprise, in addition to the thermal stabiliser, antihaze additives, 2-EHN, corrosion inhibitors, lubricity improvers, cold flow improvers, or icing inhibitors.
In one aspect the thermal stabiliser composition may comprise a mixture of compounds of the formula Hxe2x80x94R1, Hxe2x80x94R1xe2x80x94R1xe2x80x94H and/or Hxe2x80x94R1xe2x80x94H. In this aspect, preferably the R1:H ratio in the composition is 1-1.5:1 or 1-1.2:1 or approximately 1:1
The present invention will now be described in further detail in the following examples.